AlGaInP-based materials grown on lattice-matched GaAs substrates and GaN-based materials grown on sapphire or SiC substrates have led to major advances in high-brightness LEDs. That LED produces high brightness and posses complete visible spectrum to make solid-state lighting possible. The advancement of LED technology is attributed to the developments of advanced epitaxial growth technologies such as molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). The conventional growth techniques such as liquid phase epitaxy (LPE) and hydride vapor phase epitaxy (HVPE) were not suitable for growing AlGaInP crystal layer. The advanced epitaxial growth technologies enable the formation of high-quality alloy of III-V materials. They facilitate band gap engineering such as heterostructure and multiple quantum wells (MQWs) structure, which in turn increase the internal quantum efficiency and produce more light output. However, some other technical issues such as current spreading, light extraction efficiency, and heat dissipation must be resolved in order to obtain high-brightness LEDs (high wall-plug efficiency and large power output).
The most popular technique to solve the current spreading problem is presented in the U.S. Pat. No. 5,008,718. The LED structure cited in this patent is illustrated in FIG. 1A. Epitaxial layers are grown on the top of the n-GaAs substrate 10 in the following order: n-type AlGaInP cladding layer 13, active layer 14 in double heterostructure with p-type AlGaInP cladding layer 15 over the active layer and then a thick window layer GaP 19 (15-60 μm). The electrodes 21 and 22 are deposited on both sides of the structure. The advantages of using the window layer GaP 19 are that it is transparent and highly electrically conductive. But, its drawback is the extremely high thickness, which results in increasing manufacturing cost. The thick window layer is also not suitable for some device configurations—such as resonant cavity LEDs (RCLEDs) and creating photonic bandgap (PBG) in LED devices.
Alternatively, a conductive transparent material—Indium Tin Oxide (ITO) is studied and applied as a current spreading layer. FIG. 1B and FIG. 1C show the prior art of the LEDs with ITO current spreading layer. The ITO layer 20 in FIGS. 1B and 1C replaces the GaP layer 19 in FIG. 1A to serve as current spreading layer. FIG. 1B structure was disclosed in U.S. Pat. No. 5,481,122. The epitaxial structure of FIG. 1B is same as that in FIG. 1A except that a p-type contact layer 16 is inserted between the ITO layer 20 and cladding layer 15. The transmission coefficient of ITO layer 20 is about 90% in the visible range. The electrical resistivity of n-type ITO (around 2˜5×10−4 Ω-cm) is 100 times smaller than that of p-type GaP. However, a Schottky contact is formed between the ITO layer 20 and p-type contact layer 16. It degrades performance of the LEDs.
FIG. 1C shows the prior art disclosed in U.S. Pat. No. 6,580,096. Compared to the FIG. 1A, a Distributed Bragg Reflector (DBR) layer 12 is added between the layer 13 and the substrate 10 to reduce the absorption of light in the absorbed substrate 13. There are two lightly p-doped window layers 17 (GaP) and 18 (GaAs) to be added between the ITO layer 20 and the p-type cladding layer 15. The layer 17 is used to form an ohmic contact and to facilitate current spreading. The ohmic contact issue is perhaps resolved by such structure. However, the process is much more complicated and current spreading is still an issue due to the lateral contact.
For GaN material, the semi-transparent p-type ohmic contact NiO/Au (transparency is about 60%) is used as current spreading. But it suffers from low transmission. FIG. 2 shows the prior art LED presented in the paper Semicond Sci. Technol. 18 (2003) L21-L23. An ITO layer 117 is deposited on the GaN-based LED structure, which contains in the following order: sapphire substrate 110, a thin GaN nucleation layer 111, n-type GaN cladding layer 112, active layer 113, p-type cladding layer AlGaN 114, and p-type GaN contact layer 115. The electrodes 121 and 122 are fabricated on the ITO layer 117 and n-GaN 112, respectively. The major drawback is the Schottky contact formed between ITO 117 and the p-type GaN contact layer 115; such contact causes reliability problems.